This application is a continuation of U.S. patent application Ser. No. 09/518,981, filed Mar. 3, 2000 now U.S. Pat. No. 6,645,206, which is a continuation of U.S. patent application Ser. No. 09/489,317, filed Jan. 21, 2000, now U.S. Pat. No. 6,375,655, issued Apr. 23, 2002, which is a continuation of U.S. patent application Ser. No. 09/209,602, filed Dec. 11, 1998, now abandoned, which is a continuation of U.S. patent application Ser. No. 09/157,735, filed Sep. 21, 1998, now abandoned, which is a continuation of U.S. patent application Ser. No. 09/111,197, filed Jul. 7, 1998, now abandoned, which is a continuation of U.S. patent application Ser. No. 08/882,611, filed Jun. 25, 1997, now abandoned, which is a continuation of U.S. patent application Ser. No. 08/835,306, filed Apr. 7, 1997, now abandoned, which is a continuation of U.S. patent application Ser. No. 08/413,353, filed Mar. 30, 1995, now U.S. Pat. No. 5,669,909, issued Sep. 23, 1997, which is a continuation-in-part of U.S. patent application Ser. No. 08/411,017, filed Mar. 27, 1995, now U.S. Pat. No. 5,782,919, issued Jul. 21, 1998.
The number of spinal surgeries to correct the causes of low back pain has steadily increased over the last several years. Most often, low back pain originates from damage or defects in the spinal disc between adjacent vertebrae. The disc can be herniated or can be suffering from a variety of degenerative conditions, so that in either case the anatomical function of the spinal disc is disrupted. The most prevalent surgical treatment for these types of conditions has been to fuse the two vertebrae surrounding the affected disc. In most cases, the entire disc will be removed, except for the annulus, by way of a discectomy procedure. Since the damaged disc material has been removed, something must be positioned within the intra-discal space, otherwise the space may collapse resulting in damage to the nerves extending along the spinal column.
In order to prevent this disc space collapse, the intra-discal space is filled with bone or a bone substitute in order to fuse the two adjacent vertebrae together. In early techniques, bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebrae, and the spinal column was stabilized by way of a plate or a rod spanning the affected vertebrae. With this technique once fusion occurred the hardware used to maintain the stability of the segment became superfluous. Moreover, the surgical procedures necessary to implant a rod or plate to stabilize the level during fusion were frequently lengthy and involved.
It was therefore determined that a more optimum solution to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, most optimally without the need for anterior or posterior plating. There have been an extensive number of attempts to develop an acceptable intra-discal implant that could be used to replace a damaged disc and yet maintain the stability of the disc interspace between the adjacent vertebrae, at least until complete arthrodesis is achieved. These “interbody fusion devices” have taken many forms. For example, one of the more prevalent designs takes the form of a cylindrical implant. These types of implants are represented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In these cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as represented by the Ray, Brantigan and Michelson patents. In the alternative, some of the fusion implants are designed to be pounded into the intra-discal space and the vertebral end plates. These types of devices are represented by the patents to Brantigan, U.S. Pat. Nos. 4,743,256; 4,834,757 and 5,192,327.
In each of the above listed patents, the transverse cross section of the implant is constant throughout its length and is typically in the form of a right circular cylinder. Other implants have been developed for interbody fusion that do not have a constant cross section. For instance, the patent to McKenna, U.S. Pat. No. 4,714,469 shows a hemispherical implant with elongated protuberances that project into the vertebral end plate. The patent to Kuntz, U.S. Pat. No. 4,714,469, shows a bullet shaped prosthesis configured to optimize a friction fit between the prosthesis and the adjacent vertebral bodies. Finally, the implant of Bagby, U.S. Pat. No. 4,936,848 is in the form of a sphere which is preferably positioned between the centrums of the adjacent vertebrae.
Interbody fusion devices can be generally divided into two basic categories, namely solid implants and implants that are designed to permit bone ingrowth Solid implants are represented by U.S. Pat. Nos. 4,878,915; 4,743,256; 4,349,921 and 4,714,469. The remaining patents discussed above include some aspect that permits bone to grow across the implant. It has been found that devices that promote natural bone ingrowth achieve a more rapid and stable arthrodesis. The device depicted in the Michelson patent is representative of this type of hollow implant which is typically filled with autologous bone prior to insertion into the intra-discal space. This implant includes a plurality of circular apertures which communicate with the hollow interior of the implant, thereby providing a path for tissue growth between the vertebral end plates and the bone or bone substitute within the implant. In preparing the intra-discal space, the end plates are preferably reduced to bleeding bone to facilitate this tissue ingrowth. During fusion, the metal structure provided by the Michelson implant helps maintain the patency and stability of the motion segment to be fused. In addition, once arthrodesis occurs, the implant itself serves as a sort of anchor for the solid bony mass.
A number of difficulties still remain with the many interbody fusion devices currently available. While it is recognized that hollow implants that permit bone ingrowth into bone or bone substitute within the implant is an optimum technique for achieving fusion, most of the prior art devices have difficulty in achieving this fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of these devices are not structurally strong enough to support the heavy loads and bending moments applied at the most frequently fused vertebral levels, namely those in the lower lumbar spine.
There has been a need for providing an interbody fusion device that optimizes the bone ingrowth capabilities but is still strong enough to support the spine segment until arthrodesis occurs. It has been found by the present inventors that openings into a hollow implant for bone ingrowth play an important role in avoiding stress shielding of the autologous bone impacted within the implant. In other words, if the ingrowth openings are improperly sized or configured, the autologous bone will not endure the loading that is typically found to be necessary to ensure rapid and complete fusion. In this instance, the bone impacted within the implant may resorb or evolve into simply fibrous tissue, rather than a bony fusion mass, which leads to a generally unstable construction. On the other hand, the bone ingrowth openings must not be so extensive that the cage provides insufficient support to avoid subsidence into the adjacent vertebrae.
Another problem that is not addressed by the above prior devices concerns maintaining or restoring the normal anatomy of the fused spinal segment. Naturally, once the disc is removed, the normal lordotic or kyphotic curvature of the spine is eliminated. With the prior devices, the need to restore this curvature is neglected. For example, in one type of commercial device, the BAK device of SpineTech, as represented by the patent to Bagby, U.S. Pat. No. 4,501,269, the adjacent vertebral bodies are reamed with a cylindrical reamer that fits the particular implant. In some cases, the normal curvature is established prior to reaming and then the implant inserted. This type of construct is illustrated in FIG. 1 which reveals the depth of penetration of the cylindrical implant into the generally healthy vertebrae adjacent the instrumented discal space. However, this over-reaming of the posterior portion is generally not well accepted because of the removal of load bearing bone of the vertebrae, and because it is typically difficult to ream through the posterior portion of the lower lumbar segment where the lordosis is greatest. In most cases using implants of this type, no effort is made to restore the lordotic curvature, so that the cylindrical implant is likely to cause a kyphotic deformity as the vertebra settles around the implant. This phenomenon can often lead to revision surgeries because the spine becomes imbalanced.
In view of these limitations of the prior devices, there remains a need for an interbody fusion device that can optimize bone ingrowth while still maintaining its strength and stability. There is further a need for such an implant that is capable of maintaining or restoring the normal spinal anatomy at the instrumented segment. This implant must be strong enough to support and withstand the heavy loads generated on the spine at the instrumented level, while remaining stable throughout the duration.